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Pressure-Sensitive Adhesives for Decals and Labels

(October 2012) posted on Mon Oct 01, 2012

This article talks about adhesives, looks at various polymer chemistries and curing options available, and provides information.

By Julia Goldstein, Ph.D.

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Factors affecting adhesive selection
When selecting an adhesive, it is important to consider both the label material and the type of surface to which it will be bonded. Adhesion of the PSA to the label varies with label material. For example, labels printed on vinyl film stock may include plasticizers that reduce adhesion.
Aspects of the label production process, such as ink thickness, may also affect adhesive needs. To minimize costs it is usually best to keep adhesive thickness as low as possible, but rough substrate surfaces require thicker adhesives than do smooth surfaces for adequate performance. Curved surfaces require stronger adhesion to avoid peeling or buckling of the label.
Shear strength, the internal in-plane strength of the adhesive, is an important variable. If it is too low, labels may slide or peel off the surface. If adhesive shear strength is too high, however, the label may not be able to be removed cleanly if it needs to be replaced. Peel strength is related to shear strength and reflects the force needed to debond an adhesive from a surface. This needs to be relatively low for temporary applications and higher for permanent installations, but in any case it must be high enough to withstand forces experienced during the expected lifetime of the label. These forces may range from product handling to wind, depending on the application.
Environmental conditions in the field are a major factor in adhesive selection. What temperature range will the product expect to experience? Will it be installed in a dry or humid environment? Will it be exposed to water? How long does the label need to last? Will it need to be repositioned during use? All these factors affect choice of adhesive, and it may be necessary to balance trade-offs in properties to find the best fit for a given application.
Some applications are especially demanding. One example is advertising on buses (Figure 1), a lucrative, growing market. This application requires large graphics on a flexible substrate that adheres well to the contours of the bus and can be repositioned before final placement. The decal needs to withstand a harsh outdoor environment that may include freezing to very hot temperatures, rain, wind, sunlight and impact from road debris. At the same time, it needs to be removed from the vehicle without damaging the paint once the promotion has ended. These requirements will probably dictate using a solvent-based product.
Under-hood vehicle applications are another example that requires solvent-based PSAs. The combination of heat and chemical exposure would be likely to destroy many other adhesives. Silicone adhesives are a good choice for other high temperature applications, such as electronic components that need to undergo thermal processing after labeling.
Some temporary applications require PSAs that can retain their adhesive properties after being repositioned multiple times, while others, such size labeling on textiles, require that the label adhere until it reaches the end customer. In either case, ease of removal is more important than a strong bond. A specialty water-based acrylic with relatively large, spherical particles in suspension, as opposed to the emulsions common to most water-based PSAs, can work well for these applications. Silicones formulated for low adhesion are another option.
On the opposite side of the spectrum are adhesives that need to adhere securely and stay on. Some of these have tamper-resistant security features, (Figure 2) such as patterned residues that indicate evidence of tampering.


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